Furnace for heat-treating objects under high pressure

ABSTRACT

A furnace for treating material at high pressure and high temperature includes a pressure chamber and a cylindrical sheath within and spaced from the pressure chamber. The sheath is formed of three concentric annular walls forming between them substantially closed inner and outer shells. The interior of the inner shell constitutes the furnace chamber. The spaces between the walls are filled with insulating material, and the spaces between the walls open into the furnace chamber and the space between the pressure chamber and the sheath. The inner annular wall is formed of a material having a smaller coefficient of expansion than the material of the other annular walls.

United States Patent 1 I l l Inventor Hus Lundstrom {56] Relerences Cited Robsmfvrs- 5mm UNITED STATES PATENTS g 2533:2 3,240,479 3/l966 Shea. Jr. etal 263/41 Patented Aug. It I971 Primary Examiner-John J. Camby Assignee Allmann Svensh Electriska Alttlebolaget Attorney-Jennings Bailey, .l r.

Vastem. Sweden Priority Sept. 12. I968 Sweden ABSTRACT: A furnace for'treatrng material at high pressure "358/69 and high temperature includes a pressure chamber and a cylindrical sheath within and spaced from the pressure chamber. The sheath is formed of three concentric annular walls forming between them substantially closed inner and gggggg tgg OBJECTS outer shells. The interior of the inner shell constitutes the fur- 8 Cm 3 D F nace chamber. The spaces between the walls are filled with insulating material, and the spaces between the walls open into U.S.Cl 263/41 the furnace chamber and the space between the pressure Int. Cl F27b 5/00 chamber and the sheath. The inner annular wall is formed of a Field of Search 263/40. 4|, material having a smaller coefficient of expansion than the 42 material of the other annular walls.

:)===7 It" Y 3" 4A #3 v 5.1: \v {i a: K

Ian-P I W t w a t I 2o=- 30 F 's.

PATENTEU AUG] 0 mn Fig. 3

z mmww FURNACE FOR HEAT-TREATING OBJECTS UNDER HIGH PRESSURE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cylindrical, preferably vertical furnace for simultaneously treating material with high pressure, preferably 500 bar and above, and high temperature, up to 1500".

2. The Prior Art A furnace of this kind is earlier disclosed in the US. Pat. application Ser. No. 676,623, filed on Oct. 9, I967 and assigned to the assignee of the present application, entitled Vertical Tube Furnace for Isostatic Compression."

As an example of the field of use for furnaces of this type may be mentioned hot, isostatic compression or isostatic pressure sintering of metal powder. The powder, or a precompressed powder body, is enclosed in a gastight metal casing and sintered under pressure. By means of pressure sintering the sintering can be carried out at a lower temperature than when it is carried out conventionally at atmospheric pressure or under vacuum. In spite of the lower sintering temperature, the pressure-sintered product will be dense and porefree and has in many cases better mechanical properties, for example fracture strength, than conventional sintered products. With conventional casting of molten metal to billets, certain alloys acquire an unfavorable structure with strong segregation, and coarse-grain structure. Bodies of these alloys with extremely fine-grain structure can be manufactured starting with finegrained powder, for example manufactured by atomization, which is sintered under high pressure at comparatively low temperature.

An extremely important element for the function of the furnace is the heat-insulating sheath which separates the hot furnace chamber from the walls of the surrounding pressure chamber which must take up the forces from the compressed gas enclosed in the furnace and which must therefore be kept cool, or possibly be cooled. The insulating sheaths previously used normally consisted of two concentric metal tubes or pipes and insulating material in fibrous form packed into the annular space between the pipes. When the furnace is heated and the inner pipe which surrounds the furnace chamber itself is heated to, for example, l,300 C., and the outer pipe near the wall of the pressure chamber, which is cooled by this wall and therefore is only heated to about 50 or 100 C., the gap between the pipes decreases by up to percent and the insulating material is thus compressed. At the high temperature and high pressure prevailing near the inner sheath pipe, known insulating materials in fibrous form lose a considerable portion of their resilience and do not therefore return to their original shape when the furnace is cooled and the inner sheath pipe shrinks. A gap is thus formed between the inner pipe and the insulating material, and sometimes radial cracks in the insulating layer. The gas usually used, argon, which at the highoperating pressure used has extremely high density but at the same time very low viscosity, only 4-5 times that of air at atmospheric pressure, is thus very mobile. The gas convection in the gap formed between the inner sheath pipe and the insulation and in the cracks is thus intensive. The insulating capacity of the sheath may therefore be reduced to such an extent that the insulation between the sheath pipes must be repacked or even entirely replaced every time the furnace is used.

SUMMARY OF THE DISCLOSURE According to the invention the problems connected with furnaces previously used are avoided by insulating the furnace chamber itself from the pressure chamber in such a way that the heat-insulating sheath is constructed of three tubular parts arranged concentrically inside each other, the inner part consisting of a material which has a smaller coefficient of expansion than the material in the surrounding parts. The tubular parts are suitably made of sheet metal and the annular cells formed between the parts are filled with insulating material without closed pores, preferably in fibrous form, of aluminum oxide, silicon oxide or carbon base. Certain other fine-grained insulating materials of aluminum oxide or magnesium oxide base are also possible. The gap between the inner and the central tubular sheath part is chosen so that the absolute expansion of the sheath parts at temperature equality is approximately equal, both when the furnace is cold and when it is hot. The strain on the insulating material between will therefore be minimum. Suitable materials for the inner and central tubular parts are molybdenum and austenitic stainless steel, respectively, but other combinations of material are of course also feasible. It is advisable to choose the gaps between the three tubular parts so that the temperature of the central part does not exceed 700 C. The outer insulating layer is then not subjected to such high temperatures that its resilience is reduced. Any compression of the outer insulating layer is thus without importance since the insulating material expands again when the size of the gap increases. The material in the outer tubular part may thus be chosen arbitrarily. In order to eliminate the risk of compression of the inner insulating layer upon a rapid heating of the furnace chamber due to delayed heating of the central tubular part, a heating element can be arranged near the central tubular sheath part so that this can be heated so that this and the inner sheath part expand completely uniformly and the gap is therefore held constant. When molybdenum is used forthe inner sheath part and austenitic stainless steel for the central part, the same expansion is obtained if the insulating layer is chosen so that the central part has a temperature of 450 C. when the inner part has a temperature of l,300 C. The more uniform axial extension of lthe sheath parts also facilitates sealing between the cellsformed i the sheath and the surroundings. This also contributes to decreasing the convection and improving the furnace. In furnaces constructed according to the invention the insulating sheath retains its insulating properties for a long time, thusoffering great technical and economic profit.

BRIEF DESCRIPTION OF THE DRAWINGS 1 from the pressure-absorbing container, and 5 FIG. 3 shows on a larger scale a detail of the upper part of the furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The pressure chamber I consists of a thick-walled cylindrical steel tube 2, around which a wound sheath 3 of strong, cold-rolled steel wire has been wound under prestressing. The winding causes considerable stresses in the wall so that the cylinder can withstand an internal overpressure of more than 3,000 bar and can also take up axial forces generated by ,the pressure medium on the bottom and lid. Argon is used as pressure gas. Between the wound sheath 3 and an outer metal casing 4 is an annular gap 5 for coolant which is led in through the inlet 6 at the bottom of the pressure chamber and out through the outlet 7 at the upper part of the chamber. The ends of the cylinder are closed at the bottom by a plug 8 and at the top by a threaded lid 9. The bottom plug 8 supports the furnace unit 10 enclosed in the pressure chamber and has electrical through bushings II to feed the heating elements for heating the furnace and bushings I2 for thermoelement conductors and also a tube for the pressure gas, not shown. The furnace unit I0 comprises a heating-insulating sheath I3 which surrounds the actual furnace chamber l4. In the furnace chamber are electrical heating elements in the form of three loops l5, l6 and I7, suspended on the inside by a number of fireproof earthenware pipes 18. The loops are connected through the bushings 11 to a current source, not shown. Between the sheath l3 and the earthenware pipes 18 is an annular gap 19 for conductors. Of course the heating elements could alternatively be arranged on the outside of the pipes or between two concentric pipes. The heat-insulating sheath 13 which surrounds the furnace chamber 14 is constructed of three concentrical metal tubes 20, 21 and 22 which, together with end plates 23 and 24, form two completely separate annular cells 25 and 26 which are filled with fibrous insulating material. In order to equalize the pressure the cell 25 communicates with the furnace chamber 14 through the opening 27 in pipe at the upper part of the sheath and the cell 26 with the space between the sheath and the tube 20 through the opening 28 in pipe22 at the lower part of the sheath. The pressure-equalizing openings may consist of annular gaps 29 as illustrated in FIG. 3. The sheath 13 is provided with an insulating bottom 30 and lid 31 in the form of cells provided only with pressure-equalizing openings 32 and 33. The lid and bottom are also filled with insulating material. The furnace chamber 14 communicates with the space outside the insulating sheath 13 through an opening 35 in the lower part of the furnace. The inner tube 20 is made of a material having a lower coefficient of thermal expansion than that of the central tube 21. The tubes may, for example, be made of molybdenum and austenitic stainless steel, respectively. The thickness of the cells, that is the thickness of the insulation, is chosen depending on the coefficients of thermal expansion in such a way that the heat current radially out through the sheath gives the central tube such a temperature that the tubes 20 and 2] expand equally and therefore the radial measurement of the cell does not noticeably alter. In this way the insulating material heated to a high temperature in the cell is prevented from being compressed and damaged. The insulating layer thus retains its insulating properties for a long time. In order to achieve exactly uniform expansion of the tubes 20 and 2] even when the furnace chamber 14 is heated extremely rapidly it may be necessary during the heating period to heat the tube 21 with separate heating elements depending on the temperature of the tube 20.

If the radial measurements of the cells 25 and 26 are selected so that the temperature of the tube 21 never exceeds 500-700 C. depending on the material, the insulating material in the gap 26 will not lose its resilience. Thus varying expansion of the tubes 21 and 22 and consequent compression of the insulating material will therefore not deteriorate the insulating capacity of the sheath to any appreciable extent since the insulating material has not lost its ability to follow the thermal movement of the chamber. The choice of material in the outermost tube 22 is thus in practice irrelevant.

The invention is of course not limited to the embodiment shown in the drawings. Many variations are feasible within the scope of the following claims. From the safety point of view it is suitable at high pressures to use a pressure chamber with a high-pressure cylinder having a completely smooth inner surface and inwardly projecting, piston-shaped end closures. The pressure chamber is then inserted in a press stand which takes up the axial forces operating on the end closures.

I claim:

1. Cylindrical furnace for treating material with high temperature in a gaseous atmosphere under high pressure, comprising pressure chamber, cylindrical sheath within and spaced from the pressure chamber, the interior of which constitutes a furnace chamber and which is composed of at least three annular walls fonning between them substantially closed inner and outer cells comprising insulating material and provided with a pressure-equalizing opening from at least one of the cells to the outside thereof, in which the inner annular wall consists essentially of a material having a smaller coefficient of expansion than the material in the other annular walls.

2. Furnace according to claim 1, in which the annular walls are constructed of sheet metal.

3. Furnace according to claim 2, in which the cells formed between the walls are filled with insulating material.

4. Furnace according to claim 3, the inner and the middle wall and between the middle and the outer wall are spaced apart by such distances that the absolute expansion of the inner and the middle sheath parts is approximately equal in the absolute measurements. i

5. Furnace according to claim 2, in which the inner wall is constructed of ferritic steel and the middle wall of austenitic stainless steel.

6. Furnace according to claim 2, in which the inner wall is constructed of molybdenum and the middle wall is constructed of austenitic stainless steel.

7. Furnace according to claim 2, in which an element is arranged near the middle wall to heat this wall.

8. Furnace according to claim 1 having pressure-equalizing openings between the inner cell and the furnace chamber and between the outer cell and the space outside the sheath. 

1. Cylindrical furnace for treating material with high temperature in a gaseous atmosphere under high pressure, comprising pressure chamber, a heat-insulating cylindrical sheath within and spaced from the pressure chamber, the interior of which constitutes a furnace chamber and which is composed of at least three annular walls forming between them substantially closed inner and outer cells comprising insulating material and provided with a pressure-equalizing opening from at least one of the cells to the outside thereof, in which the inner annular wall consists essentially of a material having a smaller coefficient of expansion than the material in the other annular walls.
 2. Furnace according to claim 1, in which the annular walls are constructed of sheet metal.
 3. Furnace according to claim 2, in which the cells formed between the walls are filled with insulating material.
 4. Furnace according to claim 3, the inner and the middle wall and between the middle and the outer wall are spaced apart by such distances that the absolute expansion (at temperature equality) of the inner and the middle sheath parts is approximately equal in the absolute measurements.
 5. Furnace according to claim 2, in which the inner wall is constructed of ferritic steel and the middle wall of austenitic stainless steel.
 6. Furnace according to claim 2, in which the inner wall is constructed of molybdenum and the middle wall is constructed of austenitic stainless steel.
 7. Furnace according to claim 2, in which an element is arranged near the middle wall to heat this wall.
 8. Furnace according to claim 1 having pressure-equalizing openings between the inner cell and the furnace chamber and between the outer cell and the space outside the sheath. 